Stripe semiconductor lasers are well known devices in which the injection of charge carriers across one or more semiconductor junctions results in stimulated emission. Mirrored surfaces on the device are provided to form a cavity in which the stimulated emission will produce lasing when the injected current density is above a certain threshold level. By constructing the device with multiple layers of varying band-gap materials, the region in which lasing occurs may be confined, in the direction normal to the junction plane, to a well-defined active layer. The active lasing region may be limited laterally by restricting charge injection to a stripe extending between the mirrored surfaces.
An ideal device would require a low threshold current density while exhibiting linearity in the relation between light output and current above the threshold. High output power and stable intensity distribution patterns are also desirable.
Devices known in the prior art generally produce more than one lateral mode of oscillation in the junction plane at high driving currents. This is undesirable because it leads to non-linearities, commonly called "kinks", in the relation between light output and current, and complex and unstable far-field intensity distribution patterns. The severity of this problem increases with increasing stripe width.
Some prior devices have employed a narrow stripe, e.g., less than 10 .mu.m, to avoid kinks. However, a narrow stripe produces a significant detrimental increase in the threshold current density, and reduces the attainable output power of the device.
Another prior attempt to eliminate unwanted transverse oscillation modes was the so-called SWAN laser proposed by H. Kawaguchi and J. Ikegami (IEEE Journal of Quantum Electronics, Volume QE-16, No. 1, January 1980). This device used a main stripe with narrower "waists" or notches at the mirror surfaces. The abrupt changes in stripe width were intended to act as lossy filters for unwanted modes. However, these devices suffer the disadvantage that the abrupt expansion of the stripe width may cause significant conversion into unwanted higher order modes of oscillation.
Yet another prior art attempt to achieve high power laser devices is concerned with a method of stripe fabrication. The stripe is defined by diffusion of Zn, of sufficient depth to penetrate the active layer, into a completely n-type semiconductor structure. This creates a step in the refractive index across the stripe. An advantage of this technique is that an area adjacent to each mirror surface may be left undiffused, so that absorption near the mirrors is reduced. As reported by H. Yonezu, et al. (Applied Physics Letters, Volume 34, No. 10, May 1979), this has been found to increase the threshold power level which would cause catastrophic mirror damage, permitting operation of the laser at high power levels than devices with conventional stripes. However, these devices can produce unwanted lateral modes which lead to complex far-fields.
Additional prior art which may be relevant can be found in the field of passive waveguides. Passive waveguides are used for transmission of optical signals in integrated optical devices. Within this context, tapered coupling horns have been suggested as passive couplers between waveguides. These passive devices prevent mode conversion during passage of a wave from one waveguide to another. However, these waveguides are not active devices, such as lasers, and there has been no suggestion in prior art to provide a tapered width in an active semiconductor laser stripe.